the extent to which the findings obtained from an investigation conducted under particular circumstances can be generalized to other circumstances. To the extent that the circumstances of a particular investigation (e.g., patient characteristics or the manner of delivering a treatment) differ from the circumstances of interest, the external validity of the findings of that investigation may be questioned.

The extent to which the findings of a study can be generalized to other populations. For example, how far might the findings of a survey of the population of Croatia in 2003 be generalized to the population of Serbia in 2004? Such questions can usually be answered only by comparing the results of a variety of studies. See also validity.

the extent to which observed effects that are attributable to an intervention are generalizable or can be expected in other settings and populations with similar or different characteristics. See: internal validity, validity.

This is defined by Cook and Campbell (1979, p. 37) as follows: "the approximate validity with which we can infer that the presumed causal relationship can be generalized to and across alternate measures of the cause and effect and across different types of persons, settings, and times" (p. 37). See inference transferability. Back to the top

The confidence one can have about whether or not one's conclusions about the intervention can be generalised to fit circumstances, times, people, and so on, other than those of the intervention itself. A threat to external validity is an objection that the evaluation design does not allow causal inference about the intervention to be generalised to different times, places or subjects to those examined in the evaluation. See also evaluation design, internal validity, intervention, intervention logic.

External validity is a form of experimental validity Mitchell, M. & Jolley, J. (2001). Research Design Explained (4th Ed) New York:Harcourt. An experiment is said to possess external validity if the experimentâ€™s results hold across different experimental settings, procedures and participants.Brewer, M. (2000).